Process of preparing propellants



Unite States Patent PROCESS OF PREPARING PROPELLANTS John J. ONeill, Jr., Roxana, 11]., assignor to Olin Mathieson Chemical Corporation, a corporation of Virginia No Drawing. Application September 30, 1953, Serial No. 383,397

5 Claims. CI. 52-20 This invention relates generally to propellants and more particularly to a method for modifying the physical characteristics of propellent powder grains.

The gravimetric density of propellent powder grains must be regulated in order to obtain the ballistic characteristics desired in a particular cartridge. Consequently, it is often necessary to manufacture a powder having a predetermined gravimetric density for each of the many types "of cartridges manufactured, such as, for

example, by the process disclosed by Olsen, Tibbitts and Kerone in U. S. 2,027,114 or by the process disclosed by Schaefer in U. S. 2,160,626. It is not always feasible to manufacture and store a large number of powders of various gravimetric densities, however, and furthermore, in some instances such a small quantity of propellant having a gravimetric density within a certain range is required that manufacture thereof by a process which involves costly colloiding steps cannot be justified.

It is therefore an object of this invention to provide a novel and advantageous method for modifying the physical characteristics of propellent grains. Another object of the invention is to provide a method for altering the gravimetric density of hardened propellent powder grains. A further object of the invention is to modify the ballistic characteristics of a propellent powder by providing a method for lowering the gravimetric density of c-olloided smokeless powder base propellants. Still another object of the invention is to provide a method for controllably reducing the gravimetric density of a propellent powder grain.

The foregoing objects as well as others are accomplished in accordance with this invention by suspending propellent grains in a two phase system, one phase being a non-solvent medium containing a protective colloid and the other phase being a solvent for the propellent powder base which is only partially miscible with the non-solvent medium, and thereafter removing the solvent from the grain. It has been found that the reduction in gravimetric density of the grain can be most easily controlled by such a process by maintaining the suspension of the grains in the system until the solvent reaches a state of equilibrium between the grain and surrounding liquid. A reduction in gravimetric density is obtained before the state of equilibrium is reached, however, and the solvent may be removed from the grain before equilibrium, if desired. When the solvent is removed [from the grain, voids are formed therein as the grain hardens probably due to retention of non-solvent and thu a more porous product is obtained. The non-solvent medium must be agitated to maintain suspension of the grains and must contain a protective colloid such as, for example, animal glue, gum arabic, starch and the like to prevent agglomeration of the grains. Ordinarily, water is the most convenient non solvent medium but other nonsolvents for the smokeless powder base which are only partially immiscible with the solvent can be utilized.

The miscibility of the solvent with the non-solvent must be such that the solvency of the solvent for the propellent powder base is not seriously impaired or destroyed. On the other hand, the solubility of the nonsolvent in the solvent must be suflicient to insure that some non-solvent will enter the grain along with the solvent. Ethyl acetate is well suited for the solvent if the smokeless powder base is gelatinized or colloided nitrocellulose and if water is utilized as the non-solvent medium. However, any other solvent for the propellent base which is only partially miscible with the non-solvent medium, such as, for example, methyl ethyl ketone, isopropyl acetate and the like can be ordinarily utilized to advantage. For most purposes, it is preferred that the solubility of the non-solvent medium in the solvent be within the range of from about 3 percent to about 25 percent at the temperature at which the treatment of the propellent powder grains is conducted, which temperature should preferably be no lower than about 20 C. below the boiling point of the solvent in the smokeless powder base. Usually, with most solvents this temperature range is 'within about 40 and C. The amount of protective colloid required will vary with the solvent and amount thereof utilized. Ordinarily, with water as the non-solvent medium, from about 0.1 percent to 3 percent by weight protective colloid dissolved in the non-solvent medium is suificient for the purposes although greater amounts can be utilized without deleterious effect.

The general shape of propellent powder grains is not altered sufficiently by the process of this invention to seriously impair or otherwise modify the ballistic characteristics of the grain but voids are produced within the grain and the gravimetric density of the grain is reduced. The process of the invention thus permits manufacture of propellent grains in the higher gravimetric density ranges and thereafter reducing the gravimetri density of the hardened grains to other predetermined levels. This is advantageous because in practice the manufacturing process can be standardized to make powders at only one or a few gravimetric density levels and all other powders having other densities can be made therefrom.

In order better to describe and further clarify the invention, the following is a description of one embodiment thereof:

About 50 parts gelatinized substantially spherical grains of nitrocellulose base propellent powder made in accordance with the process disclosed by Schaefer in U. S. Patent 2,160,626 and having .a gravimetric or packing density of about 0.95 gram per cubic centimeter are suspended in about 400 parts water. Propellent powder grains made by the Schaefer process are substantially synmietrical. The resulting slurry is agitated to maintain the grains in suspension and is heated to about 55 C. About 3% parts animal glue are then added. About parts ethyl acetate is then sprayed into the agitated slurry of propellent powder grains at a rate such that a period of about 1 /2 hours is required for its complete addition. The slurry is heated to about 65 C. and is agitated at this temperature for about 4 hours after which time the temperature is elevated to about 99 C. to distill the ethyl acetate from the slurry. The slurry i then cooled to room temperature and the propellent grains are separated from the water by filtration. The gravimetric density of the propellent grains is about 0.35 gram per cubic centimeter.

Unless otherwise specified, all proportions of ingredients are expressed herein as parts by weight.

In the foregoing example the solvent is added to the suspension of propellent powder grains continuously at a relatively slow rate in order to avoid localized areas which contain high concentrations of the solvent capable of completely dissolving the propellent grains but if the construction of the still is such that s-uflicient agitation can be provided to insure rapid dispersion of the solvent throughout the non-solvent medium, a more rapid addition of solvent to the suspension of propellent grains can be made. However, care must be exercised to not agitate the bath sufiiciently to seriously distort the grains during processing. The solvent may be emulsified in non-solvent medium prior to its addition to the system to avoid localized concentrated areas capable of dissolving the propellent, if desired. If required to obtain desired characteristics, dinitrotoluene, 'd-ibutylphthalate, nitroglycerine or the like may be added during this treatment. If the gelatinized grains contain sufiicient deterrent when subjected to the treatment provided by this invention, obviously, no deterrent need be included in the solvent.

The amount of solvent utilized will vary with the gravimetric density of the'propellent to be treated and the ultimate gravimetric density or porosity desired but in all cases the amount used must exceed its solubility in the non-solvent medium in order to form a two phase system. With ethyl acetate as the solvent and water as the non-solvent medium, ordinarily, about 2 parts ethyl acetate for each part of propellent is sufiicient to reduce a high density substantially non-porous grain to a porous grain having a gravimetric density of less than about 0.5 gram per cubic centimeter. In examples wherein propellent grains were treated substantially in accordance with the process of the foregoing embodiment, the gravimetric density of a propellent having a gravimetric density before treatment of about 0.95 gram per cubic centimeter was reduced to about 0.8 when about A part ethyl acetate was utilized for each part of propellant. In similar treatments the gravimetric density of propellents was reduced from about 0.95 gram per cubic centimeter to about 0.65 gram per cubic centimeter with about /2 part ethyl acetate per one part propellent; about one part ethyl acetate per one part propellent reduced the gravimetric density to about 0.63 and 2 parts ethyl acetate per part of propellant reduced the gravimetric density from about 0.95 to about 0.45 gram per cubic centimeter. The maximum concentration of solvent which can be used is limited only to the extent that the grains must not become sufficiently fluid to distort or coalesce. Usually not more than about 3 hours at the desired concentration level of solvent is required for the system to reach equilibrium and further exposure time is of little or no value insofar as increasing the porosity of the propellant is concerned.

Although the foregoing embodiment has been described in detail, many variations can be made therein by those skilled in the art without departing from the spirit of the invention and the invention is not limited to such details except as set forth in the appended claims.

For example, cylindrical grains or grains of any other shape can be successfully treated in accordance with this invention. Moreover, the solvent can be removed from the suspension of propellent grains by bubbling a gas such as air through the slurry or by any other suitable means. The process is adaptable for use in treating propellants having a nitrocellulose-nitroglycerin base, a nitrostarch base or the like as well as a nitrocellulose base such as described in the foregoing.

The invention having been thus described, it is claimed and desired to secure by Letters Patent:

1. The process of reducing the gravimetric density of hardened symmetrical smokeless powder grains comprising suspending the grains in a two-phase liquid system, a non-solvent for smokeless powder base phase containing a protective colloid and a solvent for the smokeless powder base phase only partially miscible with the first said phase and having non-solvent dissolved therein, until said solvent with its non-solvent solute migrates into the grains and softens them throughout but, simultaneously, maintaining each of the grains in suspension in the nonsolvent phase as a discrete body of substantially the same shape as the original grain as it is softened, removing the solvent from the resulting soft bodies thereby hardening them into smokeless powder grains of substantially the same shape and weight but of greater porosity than the original grains, and thereafter separating the hardened grains from the non-solvent phase.

2. The process of claim 1 wherein said solvent phase has from about 3 percent to about 25 percent by weight non-solvent dissolved therein.

3. The process of claim 1 wherein the smokeless powder grains are suspended in the two-phase system until the system has reached equilibrium.

4. The process of claim 1 wherein the amount of said solvent utilized is from about A to about 2 parts by weight ethyl acetate per part propellant.

5. The process of claim 1 wherein the solvent is substantially continuously added throughout the treatment.

References Cited in the file of this patent UNITED STATES PATENTS 2,027,114 Olsen et al. Jan. 7, 1936 2,031,659 Lindsley Feb. 25, 1936 2,160,626 Schaefer May 30, 1939 2,213,255 Olsen et al. Sept. 3, 1940 2,235,298 Olsen Mar. 18, 1941 2,337,943 Silk Dec. 28, 1943 2,375,175 Silk May 1, 1945 

1. THE PROCESS OF REDUCING THE GRAVIMETRIC DENSITY OF HARDENED SYMMETRICAL SMOKELESS POWDER GRAINS COMPRISING SUSPENDING THE GRAINS IN A TWO-PHASE LIQUID SYSTEM, A NON-SOLVENT FOR SMOKELESS POWDER BASE PHASE CONTAINING A PROTECTIVE COLLOID AND A SOLVENT FOR THE SMOKELESS POWDER BASE PHASE ONLY PARTIALLY MISCIBLE WITH THE FIRST SAID PHASE AND HAVING NON-SOLVENT DISSOLVED THEREIN, UNTIL SAID SOLVENT WITH ITS NON-SOLVENT SOLUTE MIGRATES INTO THE GRAINS AND SOFTENS THEM THROUGHOUT BUT, SIMULTANEOUSLY, MAINTAINING EACH OF THE GRAINS IN SUSPENSION IN THE NONSOLVENT PHASE AS A DISCRETE BODY OF SUBSTANTIALLY THE SAME SHAPE AS THE ORIGINAL GRAIN AS IT IS SOFTENED, REMOVING THE SOLVENT FROM THE RESULTING SOFT BODIES THEREBY HARDENING THEM INTO SMOKELESS POWDER GRAINS OF SUBSTANTIALLY THE SAME SHAPE AND WEIGHT BUT OF GREATER POROSITY THAN THE ORIGINAL GRAINS, AND THEREAFTER SEPARATING THE HARDENING GRAINS FROM THE NON-SOLVENT PHASE. 